1. Field of the Invention
The present invention relates to a temperature sensor for a resistance thermometer comprising an electric measuring resistor the resistance material of which consists either of a ceramic material or of a metallic resistance material in combination with a ceramic or a mineral carrier, a protective tube comprising a closed tip and a rear end that provides an access to the inner space of the protective tube and that contains the measuring resistor closely adjacent the closed tip of the protective tube, at least one supply line that is brought out through the rear end of the protective tube, and an electrically insulating filler based on a ceramic or a mineral material that fills the space between the protective tube on the one side and the measuring resistor and its at least one supply line on the other side. A temperature sensor of that kind has been known from DE 44 24 384 C2. It comprises a measuring resistor made from a resistance material, especially from platinum, arranged on a planar substrate. A metallic protective tube, equipped with a separate test prod, is provided for receiving the measuring resistor. The test prod consists of a tube, which is closed at its forward end and which is squeezed together, while it is still empty, over its forward section to form a planar narrower portion. The planar measuring resistor, which is introduced into the narrower portion, comprises connection lines that are extended in length by connection wires brought out through the rear open end of the test prod. After the measuring resistor has been pushed in, a polysilicate-based ceramic compound is filled into the test prod so as to fill the space still available in the test prod and to thereby fully embed the measuring resistor. The test prod so prepared is then welded to a longer metallic protective tube with the tapering test prod projecting from the tube. Inside the longer protective tube the connection wires extend in an air space and are connected with the conductors of a connection cable the sheath of which is fixed in a rear end portion of the protective tube by crimping. The described temperature sensor is relatively expensive to produce and not durable enough for application in an exhaust gas system of a combustion engine.
2. Description of the Related Art
The operating conditions of the temperature sensor in the exhaust gas system of combustion engines are severe. They are characterized by high temperatures of over 600° C. up to almost 1000° C., high temperature variations, for example by temperature rises by 800° C. in only 5 seconds, by vibration and by contact with aggressive agents flowing around the sensor. The known temperature sensor cannot withstand such conditions. Rapid temperature variations, in connection with the different coefficients of thermal expansion of the metallic connection wires on the one hand and the ceramic embedding compounds on the other hand, result in stresses that act on the connection wires and that may cause the connection wires to break. To leave the largest part of the protective tube free from ceramic embedding compound, as is proposed by DE 44 24 384 C2, does not provide a solution to that problem because the exposed connection wires are at risk to be damaged by the continuous vibrations, which also present a risk for the measuring resistor.
Temperature sensors of the kind disclosed by DE 102 54 637 B4 and DE 199 22 928 A1, where a thermistor embedded in a coagulated ceramic powder, for example aluminum oxide, is connected with a mineral-insulated cable which is in direct contact with the aluminum oxide embedding compound, are connected with similar disadvantages.
DE 100 34 265 A1 discloses a temperature sensor with a measuring resistor located in a closed protective tube where the resistor, instead of being embedded in an insulating ceramic material, is enclosed in an air chamber. The connection cable consists of a mineral-insulated metal-sheathed two-core cable welded to the rear section of the protective tube for the measuring resistor. In that case, the measuring resistor, extending freely in an air chamber in the protective tube, is especially exposed to the danger of being damaged by vibrations and responds slowly, being efficiently thermally insulated by the air surrounding it. In order to overcome that disadvantage it has been known from DE 101 58 527 A1 to provide openings in the tip of the protective tube, in the area of the measuring resistor, that permit the exhaust gas to enter the inner space and to flow around the measuring resistor. While this has the result that the temperature sensor responds to temperature variations more quickly, the measuring resistor is exposed to chemical attack by the exhaust gases, tends to get soiled by deposition of unburnt fuel residues, especially soot and condensate, which leads to the additional risk that an electric shunt will build up been the supply lines of the measuring resistor. Moreover, measuring resistors intended for being exposed to the exhaust gas in an open protective tube are considerably more expensive than measuring resistors that are intended for use in closed protective tubes.